I currently try to get gcalcli working with conky. Everything looking good so far, I am just stuck on 1 error:

error: argument command: invalid choice: 'calw2'

Which makes sense. It should be calw 2 instead of calw2.

Apparently, terminal doesn't tell me which file it tried to run that contains the error. Is there a way to get a log which exactly shows which files are used and in which file the error appears running an application (like here: conky) in terminal?

The terminal program isn't intended to tell you anything about these errors - it simply provides a user interface to the shell. It would be your command-line application that should tell you where it encountered a problem.
– roaimaMar 3 '17 at 15:31